1. Field of the Invention
This invention is concerned with the purification of sulfur-contaminated gas streams. It is particularly concerned with an improved process for removing low concentrations of hydrogen sulfide contaminant in a waste gas stream by contact with a regenerable, reactive aqueous sorbent, the improvement residing in a novel treatment of sorbent degraded by excess thiosulfate content. The novel treatment recovers chemical values and mitigates disposal problems.
2. Prior Art
Low concentrations of sulfur contaminants occur in gas streams such as natural gas, coke oven gas, and the tail gas from the Claus process. These contaminants are usually undesirable for one reason or another. In particular, in the Claus process which is widely used in petroleum refineries to convert by-product hydrogen sulfide to sulfur, large volumes of waste tail gas are generated which may contain up to about 2.0 wt. % sulfur contaminant. Directly discharging this tail gas to the atmosphere results in pollution problems, and increasing numbers of communities are requiring that the contaminant level be drastically reduced prior to discharge.
A recently developed and highly effective process, known as the Stretford Process, has been used to remove low concentrations of sulfur contaminants from the gas streams. In this process, if the sulfur contaminant is not all present as hydrogen sulfide, it is first converted to this compound by the Bevon process, for example, which utilizes catalytic hydrogenation with a cobalt-molybdenum type catalyst. The hydrogen sulfide contaminated gas stream is then contacted with an aqueous alkaline sodium carbonate-bicarbonate solution that contains salts of oxyacids of pentavalent vanadium and the salts of anthraquinone and/or hydroanthraquinone disulfonic acids (hereinafter referred to simply as A.D.A. and reduced A.D.A., respectively). Substantially all of the hydrogen sulfide is converted to elemental sulfur which is recovered. This oxidation of hydrogen sulfide occurs with a concomitant reduction of the pentavalent vanadium to tetravalent form, and some reduction of the A.D.A. may also occur. A properly treated gas stream will contain less than about 0.002 wt. % residual sulfur contaminant. After separation of the elemental sulfur, the spent aqueous sorbent containing tetravalent vanadium is regenerated with oxygen gas or air to form reactive aqueous sorbent which is recycled to the sorption step. The net effect of the cycle is the indirect oxidation of the hydrogen sulfide by oxygen gas to form water and elemental sulfur which is recovered.
In practice, the described process has been found to perform its intended function of removing substantially all of the sulfur in a waste gas stream or other gas stream extremely well. Nonetheless, there is a problem associated with this process due to the fact that as much as about 5% of the sorbed sulfur contaminant is not recovered in the form elemental sulfur but, instead, forms thiosulfate and, to a lesser extent, other water-soluble oxysulfur anions, such as sulfate. The reaction path by which thiosulfate forms is not entirely clear, but some undoubtedly is formed by the direct oxidation of the hydrosulfide ion. Another contribution may be from the known reaction of sulfur with sodium carbonate. A fraction of the thiosulfate, in turn, is converted to sulfate in the presence of A.D.A. or reduced A.D.A. and the oxygen used as regenerant. Regardless of how formed, thiosulfate and related compounds accumulate in the regenerable, reactive aqueous sorbent with adverse effects. Thus, either the periodic replacement of the entire sorbent or the withdrawal of a purge stream is required to maintain the concentration of these compounds at a tolerable level in the system. In general, sodium thiosulfate levels in excess of about 25 wt. %, as the pentahydrate, are very undesirable because both A.D.A. and the vanadate salts are largely salted out at these levels. Since either complete or partial replacement of the oxysulfur anion-loaded sorbent is, in effect, a purging of the system, the term "purge stream," as used herein, is to be understood to refer to either or both modes of replacement. It will be understood that if a purge stream of sorbent is merely taken and discarded, valuable soda, A.D.A. and vanadium compounds will be lost with the removed sorbent. More importantly, however, is that the removed thiosulfate containing sorbent presents a serious disposal problem since it has a high B.O.D. (Biological Oxygen Demand) by virtue of its high concentration of thiosulfate. This disposal problem was recognized in U.S. Pat. No. 3,904,734 which provides a method in which the purge stream is decomposed in a fluidized bed before ultimate disposal. While this process eliminates the environmental problems associated with the disposal of the purge stream, it results in the loss of valuable chemicals.
In U.S. Pat. No. 3,959,452 a process is described in which a purge stream is acidified with sulfuric or phosphoric acid to decompose thiosulfate to elemental sulfur and sulfur dioxide. After acidification, the solution is blown with steam or hot flue gas to remove substantially all of the SO.sub.2 and decanted to remove floculated sulfur. The solution is then treated with calcium hydroxide to convert substantially all of the sodium sulfate or phosphonate to sodium hydroxide and to precipitate calcium sulfate or calcium phosphate, depending on the acid utilized to decompose the thiosulfate. While this process represents an improvement over purging and disposing Stretford solution, it has been found that some losses of valuable chemicals still occur since some A.D.A. is occluded in the amorphous sulfur and some vanadium is lost because of the limited solubility of calcium vanadate.
It is an object of this invention to provide an improved method for reducing the losses of vanadium and A.D.A. in the purge stream of the Stretford process.